1. Field of the Ivention
The present invention relates to an apparatus for measuring a water content of an object to be measured, wherein the object to be measured is irradiated with a reference light having a wavelength not easily absorbed therein and a measuring light having wavelength easily abosrbable therein, and the quantity of water contained in the object is calculated from the rate of the quantity of the measuring light absorbed therein with respect to the quantity of the reference light absorbed therein, and more particularly to such an apparatus further including a calibration device.
2. Statement of the Prior Art
Hitherto, such an apparatus has been known from Japanese Patent Laid-Open (Kokai) Publication No. 61(1986)-4915, for instance. According to the teachings disclosed therein, reference and measuring lights to strike upon an object, the water content of which is to be measured, are obtained from the same light source through optical filters.
Then, the reference and measuring lights are sent to a photoelectric converter through the object to be measured, and the rate of the quantity of the measuring light absorbed therein with respect to the quantity of the reference light absorbed therein is calculated on the basis on the output signals produced from that converter.
Further, the quantity of water contained in the object to be measured is calculated from the rate of the measuring light absorbed, and is indicated on a display.
When such an apparatus is used for measuring the water content of granular materials such as coal in the open air, the reference and measuring lights are absorbed in moisture present in optical paths, through which the reference and measuring lights pass, due to variations in the humidity thereof.
In consequence, it is impossible to measure the quantity of water contained in the object to be measured with high accuracy.
Another proposal has been made in Japanese Patent Kokai Publication No. 59(1984)-72047 in which the humidity of the aforesaid optical paths is detected to correct the measurement value with the thus detected humidity.
However, there is a change-with-temperature in the characteristics of the optical filters for obtaining from the same light source the reference and measuring lights to strike upon the object, the water content of which is to be measured. Even with such a proposal, therefore, it is impossible to measure, with high accuracy, the quantity of water contained in the object to be measured owing to such temperature dependence.
In the prior art, the correction characteristics of measurements have had to be manually preset by operation of a potentiometer, etc. However, this has been laborious and timeconsuming.